1. Field of the Invention
The present invention relates to an image processing technique for performing tone correction for an image.
2. Description of the Related Art
Regarding a gamma characteristic, which indicates an input-output characteristic of an image, as the gamma slope increases, the expressiveness of the tone of an output image with respect to an input image increases. The gamma characteristic functions as inverse gamma of an input-output characteristic of a television, and is determined depending on how the wide dynamic range of the natural world is to be compressed and brought into the dynamic range of a camera.
The normal gamma characteristic is set such that its slope in a lower-luminance portion of an image is steeper and its slope in a higher-luminance portion is less steep. For this reason, the higher-luminance portion is disadvantageous in terms of the expressiveness, compared with the lower-luminance portion. With a camera having such a gamma characteristic, if a high-luminance object exists in a image capturing screen and a user does not want to change the brightness of the entire image capturing screen, in general, the slope of the gamma in the high-luminance portion is increased. Thus, it is possible to darken only the high-luminance object and improve the expressiveness of the tone. However, if the tone in the high-luminance portion is adjusted in the case where the high-luminance object is a person's skin, color curving is noticeable.
Even if color curving occurs and some colors look different from those of the actual object in the case where the colors are not memory colors such as the colors of the sky, greens, and the skin, it would hardly look unnatural, and even when it looked unnatural, this unnaturalness would be tolerable in many cases. However, in the case of the memory colors, since people are highly sensitive to the color of a person's skin, it will look unnatural if there is even a small difference from the colors of the actual object or the colors in their memory. In particular, this results in a feeling that the person's skin color looks unnatural, and gives a strong impression that the camera has bad image quality.
As a technique related to the aforementioned color curving, for example, Japanese Patent Laid-Open No. 10-234052 recites that when any color of R, G, and B starts to be saturated, the output of the other colors is clipped at that point in time.
However, in Japanese Patent Laid-Open No. 10-234052 mentioned above, even if color curving is suppressed, the colors other than the saturated color are clipped at the point in time when any color of R, G, and B is saturated, and accordingly, the tone of the pixels which are equal to or greater than the clip level cannot be expressed, resulting in an unnatural image with poor plasticity. Further, there is also a method of deleting a color at the point in time when this color, which is any color of R, G, and B, is saturated in order to prevent the color from remaining and thus avoid the poor plasticity, but this method also results in an unnatural image since the color suddenly disappears.